This invention relates to decarboxylation of saturated carboxylic acids on semiconductor powders.
Applicants discussed background information relative to their invention in a paper published at 100 Journal of the American Chemical Society 5985 (1978) which paper is hereinafter referred to as "applicants' paper". In applicants' paper, different types of photoelectrochemical processes were discussed, and it was noted that semiconductor materials are of central importance in electrochemical systems which can utilize solar energy for the production of electricity or new chemical species.
In photoelectrochemical cells operating in the photovoltaic mode, the light which irradiates the semiconductor-solution interface is converted into electricity ideally with no change in the composition of the solution or the semiconductor material (See FIG. 1a in applicants' paper). The driving force in such a cell is the underpotential developed for an oxidation at the n-type photoanode (or a reduction at a p-type photocathode).
In photoelectrosynthesis the light is used to drive an overall cell reaction in a nonspontaneous direction so that the radiant energy is stored as chemical energy (e.g., in fuels) (See FIG. 1b in applicants' paper). Although earlier studies in this area were concerned with the photolysis of water (i.e., production of H.sub.2 and O.sub.2), studies of photooxidations of other solution species at n-type semiconductor electrodes have subsequently provided information about the mechanism of such photoassisted processes, and have been extended to the bulk synthesis of other chemical species.
In photocatalysis a reaction is driven in a spontaneous direction by the light; radiant energy overcomes the energy of activation of the process (See FIG. 1c in applicants' paper). Cells which operate simultaneously in the photovoltaic and photoelectrosynthetic or photocatalytic modes are also possible.
While photoredox processes in homogeneous solutions are usually inefficient, the electric field (or band bending) at the photoexcited semiconductor-solution interface causes rapid separation of the carriers and thus inhibits recombination of the highly reactive light-generated electron-hole pair. Furthermore, the primary product of electron transfer at the semiconductor-solution interface often does not suffer rapid back-donation of the electron from the electrode, as overpotentials for redox processes involving energy levels in the forbidden band gaps of the semiconductor may be considerable. Thus high quantum yields can be obtained in heterogeneous photoredox processes. Moreover, fast, irreversible chemical reactions of the solution species following the electron transfer can compete with the reverse charge transfer at the electrode. With this in mind and to extend the scope of synthetic methods at illuminated semiconductors, applicants had earlier investigated a chemical electrosynthetic reaction, the Kolbe decarboxylation of carboxylic acids.
The well known Kolbe synthesis may be illustrated in the case of ethane from sodium acetate as follows: ##EQU1## where the first two reaction products illustrated above are formed at the anode and the latter two at the cathode.
The Kolbe reaction does not appear to be an attractive one in the search for an energy-storing system, since the cleavage product, carbon dioxide, renders most simple decarboxylations exoenergetic. Moreover, a high oxidation potential is needed for initiation of the Kolbe reaction on metal electrodes and many semiconductors (e.g., ZnO, CdS) show limited stability against photodecomposition under strongly oxidizing conditions. Therefore, among the variety of investigated photoinduced oxidation processes at n-type semiconductor materials the Kolbe reaction has previously received only minor attention.
In earlier studies of the applicants, reported in 1977 at 99 Journal of the American Chemical Society 7729 (1977) applicants reported on the photoassisted oxidation of acetate ion to ethane in a acetonitrile solution at n-type titanium dioxide electrodes in both the single crystal and chemically vapor deposited polycrystalline form. In that earlier paper, applicants reported a reaction mechanism that follows that for other photoassisted oxidations at n-TiO.sub.2. There, they had reported that light of energy greater than E.sub.g causes formation of electron-hole pairs. When the potential of the semiconductor is positive of the flat-band potential, the bands are bent upward and the photogenerated holes (p+) migrate to the electrode surface while the electrons drift to the bulk of the electrode, thus preventing recombination. The holes, at energies characteristic of the valence band or low-lying surface states, are effectively strong oxidizing agents and can abstract electrons from acetate ions initiating the cascade of steps in the Kolbe reaction: EQU CH.sub.3 CO.sub.3.sup.- +p.sup.+ .fwdarw.CH.sub.3 +CO.sub.2 EQU 2CH.sub.3 .fwdarw.C.sub.2 H.sub.6
Since the photo-Kolbe reaction occurs at potentials of approximately 2.4 V more negatively than those for the oxidation at platinum, the effective efficiency of utilization of the excitation energy is high. The chemical irreversibility of the overall process prevents back-donation of electrons from the electrode and leads to an overall high efficiency for the process. The potential for the photo-oxidation of acetate is negative of that for the onset of hydrogen ion reduction in the acetate/acetic acid mixture, suggesting that photoelectrolysis with little or no external applied voltage leading to a mixture of ethane and hydrogen is possible. Furthermore, the control of current density via the light flux and the possible suppression of undesirable 2-electron oxidations by use of photoinduced electron transfer at a wide band gap semiconductor (e.g., TiO.sub.2) are also of interest with respect to the Kolbe electrosynthesis.